A medical device for use with a fluid transfer device includes a hub having an open proximal end with a frusto-conically-shaped cavity therein, a distal end and a passageway therethrough. The cavity is part of the passageway. A release element in the passageway of the hub is positioned to block fluid-tight engagement of the frusto-conically-shaped tip with the cavity of the hub. Structure is provided to release at least part of the release element, upon application of a proximally directed force on the hub, to allow the abrupt fluid-tight engagement of the tip and the cavity in the hub.